Maybe I should word that better. I don't think they're going off works alone and are focusing more on the actor's skills. While appearance is important we would also need actors that could actually play the roles convincingly.

If you compare Lumiere, Cogsworth and the Beast in Disney's 1991 animated film Beauty and the Beast to their live-action versions in Disney's upcoming live-action adaptation, you can see a clear example of cartoony and abstract characters being contorted into strange and grotesque creations that don't really resemble their original animated versions. With Gravity Falls, I suspect that any live-action adaptation will encounter a similar problem, because the more abstract an animated film or television series is, the less suited it is to being adapted into live-action (for example, just try to imagine a live-action Finding Nemo).

Any live-action adaptation of Gravity Falls would inherently lack the value animation adds to the series, and therefore would need to make up for that with deviations from the source material — including key changes to what the main characters look like. And because of that alone I'm extremely hesitant to the idea, but mainly it would just be an absolute travesty to see what happened wth the live-action adaptation of Avatar: The Last Airbender happen to Gravity Falls.

I do like talking about potential casting for the mer fun of it, but to be quite honest I am actually a little hesitant myself on the idea of a live-action Gravity Falls movie for many of the reason you have pointed out Reptile.

An Alex Hirsch thread? Finally! Now's my chance before it's too late! After much study, I have found that Hirsch is from another dimension. I can't believe it had taken me 11 years to figure it out. He was originally suppose to arrive in this dimension unnoticed, but he crashed somewhere in the vast nothingness of Siberia back in 1908. He was sent by some crazy underground cryptic Order to test our feeble minds in all sorts of skills, particularly our abilities in code-cracking. There is Hirsch hiding in every shadowy corner, jotting and recording the progress. How could he be in more than one place at once, you may ask? He is omnipotent! Or he just cloned himself. I haven't been able to fully research either theory. Studies suggest that in the year 2021, Hirsch's true form will be visible for 33.33 seconds. Don't look straight into his face, or else you'll be trapped. It's a fact that his face causes the event horizon. No...his face IS the event horizooon!
He also made this galaxy. Yup. He was cleaning out his attic, and all that junky dust formed into what you now know as the Milky Way. Praise Hirsch. He is your true creator! Don't make him mad, or else he'll pelt rocks at your planet. Asteroids as you call them. Those poor, poor, dumb dinosaurs. It was all the fault of that Kryptoapparatus! What a jerk!